The term "Holocaust" includes only the Jewish victims (6 million Jews), this term never included anyone else. So it's only right that a Jewish symbol is on a Jewish memorial. Jews are an ethno-religious group so it's logical that their symbol has some religious connotations, but as many Jews became non-religious in the last few decades and still identify with their Jewish identity, one could call it a symbol of the Jewish people as such, regardless of what they believe in now. After all, it's on the Israeli flag. I mean, that's like complaining that the 'double cross' on Hungarian and Slovak coat of arms is 'sectarian' and 'exclusionary' religious symbol, because it originates from 12th century Christian Hungarian Kingdom. One can't simply erase centuries of history and tradition just because he doesn't like what his ancestors believed in.

Anyway, I do believe that non-Jewish victims of the Nazi agression in Europe are often neglected. But this is a subject for some other discussion and it's quite complex. The fact is that the term 'holocaust' (or 'shoah') is reserved for Jewish victims only. This atheist group obviously doesn't know much about WWII history/is not interested in it (otherwise they would be familiar with the basic terminology), they just want to create noise. It's very unjust to use something like a memorial for innocent victims to promote your own ideology, but that's how these groups operate.